When a rapid neutron nuclear reactor is shut down, the power supplied does not die away instantaneously, but reduces gradually for some time before dying completely away. This residual power may be absorbed in special heat exchangers, to which the heated liquid alkaline metal is conveyed, instead of to the normal operation exchangers. Such special exchangers are however relatively expensive and are used for fairly short periods of operation. They therefore substantially increase the cost price of nuclear power stations using rapid neutron nuclear reactors.